1. Field of the Invention
This invention relates to the field of synthetic cable terminations. More specifically, the invention comprises a method for terminating a large, multi-stranded cable having at least a partially non-parallel construction.
2. Description of the Related Art
Synthetic rope/cable materials have become much more common in recent years. These materials have the potential to replace many traditional wire rope assemblies. Examples of synthetic fibers used in cables include KEVLAR, TWARON, TECHNORA, SPECTRA, DYNEEMA, ZYLON/PBO, VECTRAN/LCP, NYLON, POLYESTER, GLASS, and CARBON (fiber). Such fibers offer a significant increase in tensile strength over traditional materials. However, the unique attributes of the synthetic materials can—in some circumstances—make direct replacement of traditional materials difficult. This is particularly true for larger cables. As those skilled in the art will know, it is not practical to simply scale up termination technology used in small synthetic cables and expect it to work on large synthetic cables.
This disclosure will employ consistent terminology for the components of a synthetic cable. The reader should note, however, that the terminology used within the industry itself is not consistent. This is particularly apparent when referring to cables of differing sizes. A component of a small cable will be referred to by one name whereas the analogous component in a larger cable will be referred to by a different name. In other instances, the same name will be used for one component in a small cable and an entirely different component in a large cable. In order to avoid confusion, the applicants will present a naming convention for the components disclosed in this application and will use that naming convention throughout. Thus, terms within the claims should be interpreted according to the naming convention presented.
First, the terms “rope” and “cable” are synonymous within this disclosure. No particular significance should be attached to the use of one term versus the other. The smallest monolithic component of a synthetic cable will be referred to as a filament. A grouping of such filaments will he referred to as a “strand.” The filaments comprising a strand may be twisted, braided, or otherwise gathered together. Strands are grouped together to form a cable in one or more stages. As an example, strands may be grouped together into “strand groups” with the strand groups then being grouped together to form a cable. Additional layers of complexity may be present for larger cables. A particularly large cable might be grouped as follows (from smallest to largest): filament, strand, strand group, strand group group, cable. The term “strand group” is generally only used for massive cables. However, it is not used consistently in the industry. In any event, the term “strand” is always used to indicate some portion of a cable that is less than the entire cable itself. Many different subdivisions of a cable may appropriately be called a strand.
The filaments and strands will normally be tension-carrying elements. However, some cables include other elements, such as one or more strands intended to measure strain. The invention is by no means limited to cables including only tension-carrying elements.
The process of grouping filaments, strands, or strand groups together commonly involves weaving, braiding, twisting, or wrapping. For example, it is common to wrap six twisted strands around a twisted straight “core” strand in a helical pattern. Some examples of cable construction will aid the reader's understanding.
FIG. 1 shows a prior art cable 10 comprised of seven strands 12. A single “core” strand is placed in the center. Six outer strands are then helically wrapped about the core strand to form the pattern shown. FIG. 2 shows an individual strand 12. Strand 12 is comprised of many individual filaments 16 which are also wrapped in a helical pattern. Jacket 14 surrounds and encapsulates the filaments in this particular example. A jacket is included on some strands and not on others. A jacket may assume many forms. Some are an extruded covering. Some are a helical wrapping. Still others are a braided or woven layer of filaments which surround the core filaments.
The scale of the strand and filaments of FIG. 2 is significant to understanding the present invention. Each individual filament is quite small, having a diameter which is typically less than the diameter of a human hair. The filaments shown in FIG. 2 are larger in comparison to the overall cable diameter than is typical for synthetic cables. The larger filament diameter is shown for purposes of visual clarity. Strand 12 in FIG. 2 might have an overall diameter between 1 and 15 mm. Several such strands may be grouped directly together to make a cable as shown in FIG. 1.
FIG. 3 shows a cable having three levels of grouping. Filaments are grouped together to make strands 12. Seven strands are then grouped together to form a strand group 19. Seven such strand groups 19 are then grouped together to form cable 10. As explained previously, the term “strand group” may also be referred to as a “strand” (since it is a subdivision of the entire cable). Note that the entire cable may be encompassed by a jacket 14. As for the smaller levels, the jacket may assume many forms.
The reader will note that the cable as a whole has central axis 30 running down its center. The strands 12 generally run in the direction of central axis 30 but they are not all parallel to it. For the example of FIG. 3, each strand 12 is wrapped in a helical fashion (except the core strand of each strand group). Strand groups 19 are shown as being nearly parallel to central axis 30. However, in other examples the strand groups may be helically wrapped around the central axis as well. In still other examples they may be braided or woven. In the context of this disclosure, the term “non-parallel” simply means that a strand is not parallel to the cable's overall central axis. The strand may, on average, follow the central axis. But, at any given point a normal vector of the strand's cross section is not parallel to the overall central axis of the cable. The strand follows a curved path (formed by processes such as twisting, braiding, etc.)
Most prior art cables made using synthetic filaments are relatively small. The example of FIG. 1 might have an overall diameter between 1 mm and 15 mm. Of course, the individual filaments within the strands are very small. A synthetic filament is analogous to a single steel wire in a bundled wire rope. However, the individual synthetic filament behaves very differently in comparison to a piece of steel wire. When such a comparison is made, the synthetic filament is: (1) significantly smaller in diameter; (2) much less stiff (having very little resistance to buckling and quite vulnerable to bending-induced deformation); and (3) slicker (The synthetic strand has a much lower coefficient of friction). Of these differences, the lower stiffness inherent in the use of synthetic filaments is the most significant.
Another significant difference between the individual filaments comprising a synthetic cable and the steel wires commonly used in wire ropes is the scalability of the most basic component. Steel wire is typically created by a drawing process. This allows the wire to be created in a wide range of sizes. A small diameter steel wire is used to make a small wire rope and a large diameter steel wire is used to make a large wire rope. The most basic component of a wire rope—the steel wire—may be easily scaled to match the size of the wire rope. This is not true for the use of synthetic filaments. A synthetic filament having suitable properties is limited to a fairly narrow range of diameters. Thus, the basic component of a synthetic cable is not scalable. A very fine filament must be used for a small synthetic cable and essentially the same size of filament must be used for a large synthetic cable.
In order to carry a useful tensile load any cable material must have a termination (typically on its end but in rare occasions at some intermediate point). The word “termination” means a load-transferring element attached to the cable that allows the cable to be attached to something else. A portion of the cable itself will typically lie within the termination. For a traditional cable made of steel wire, a termination is often created by passing the cable around a thimble (with an eye in the middle) and clamping or braiding it bank to itself. For higher load situations, the end of a wire rope may be terminated using a socket. The word “socket” in the context of wire rope terminations means a generally cylindrical steel structure with a conical cavity. The sheared end of the wire rope is placed in the cavity and the individual wires are then splayed apart. Molten zinc is then poured into the cavity and allowed to solidify (Epoxy resins and other synthetic materials may now be substituted for the zinc) Such a socket commonly includes an eye or other feature allowing the cable to be attached to an external component.
A variation on the socket approach has been successfully employed for synthetic cables having a relatively small diameter. The device actually placed on the end of a synthetic cable in order to create a termination is commonly referred to as an “anchor.” FIGS. 4-6 show one process for creating a termination on a synthetic cable using such an anchor.
In FIG. 4, cable 10 has been cut to a desired length. The individual strands are very flexible. Accordingly, binder 20 has been added some distance back from the cut end. This distance is labeled “set-back distance” 36. The set-back distance is roughly equal to the length of filaments which will be placed within the cavity in a termination. Free filaments 26 are unbound and free to flex. The binder wraps around the cable and primarily helps it retain a compressed or otherwise bound cross section to better control filament movements during processing. The use of a binder is preferred.
Splayed filaments 34 are placed within the cavity of an anchor. They are generally splayed apart before they are placed in the anchor cavity, but they may also be splayed apart after they are placed in the anchor cavity. In a traditional potting process, the cavity is then filled with a liquid potting compound. The term “potting compound” means any substance which transitions from a liquid to a solid over time. A common example is a two-part epoxy. The two epoxy components are mixed and poured or injected into the cavity before they have cross-linked and hardened. Other compounds are cured via exposure to ultraviolet light, moisture, or other conditions.
FIG. 5 shows a section view through such a termination after the potting compound has hardened into a solid. Anchor 24 includes a tapered cavity through its center. A length of filaments is locked into potted region 28 by the hardened potting compound. Free filaments 26 rest outside the anchor.
In the example of FIG. 5, a single strand has attached to a single anchor. This is not the only possibility and the invention is not limited to just this one possibility. It is possible to attach multiple strands to a single anchor (such as by potting a three-strand twisted rope into a single anchor). This would be a connection between a single anchor and a strand group. It is also possible to divide a single strand into a plurality of substrands and attach each of the sub-strands to an anchor. Thus, one strand could be attached to two or more anchors.
An anchor attached to a cable typically includes a load-transmitting feature designed to transmit a tensile load on the cable to some external component. This could be a hook or an external thread. As such features are well understood in the art, they have not been illustrated.
Those skilled in the art will know that an anchor may be attached to a cable by many means other than potting. Another well-known example is a frictional engagement where the splayed strands are compressed between two adjacent surfaces. A “spike and cone” connection, sometimes referred to as a “barrel and socket” connection, attaches an anchor to a cable using this approach. An example of such a connection is shown in FIG. 39 (and described in more detail subsequently).
Another approach to creating a termination is to cast a composite “plug” on the end filaments of a cable. The plug is preferably cast in a desirable shape that allows it to be easily attached to an external component.
The cable of FIG. 5 is relatively small—having a diameter between 1 mm and 10 mm. The potting process and other mechanical termination means work fairly well for such cables. FIG. 6 shows a perspective view of a completed assembly where the anchor is attached via potting. Anchor 24 and potted region 28 collectively form termination 32 on one end of cable 10. The reader should note that cable 10 is parallel to anchor 24. The filaments within the cable may be non-parallel (They may for example be helically wrapped or braided). However, the overall centerline of the cable is parallel to the centerline of the anchor. This constraint is significant, because the ultimate strength of synthetic cables decreases significantly if the freely flexing portion of the cable is angularly offset with respect to the anchor. The desired alignment becomes a more difficult problem for larger cables—as will be seen.
FIG. 7 shows a larger cable 10. The example shown has a diameter of 50 mm (Even larger synthetic cables are presently in use). Braided jacket 18 surrounds and encloses smaller strand components and strand group ultimately individual filaments. Binder 20 is placed around the cable and the jacket is removed for loose portion 22. For a cable of this size, loose portion 22 is comprised of tens of thousands to millions of individual filaments. The filaments are very flexible, having a stiffness that is similar to human hair. The loose portion is akin to the head of a mop—though it is in reality even less organized and much more flexible than the head of a mop.
It is very difficult to employ the prior art termination process for the synthetic filament cable shown in FIG. 7. FIG. 8 shows an anchor 24 which is sized for this cable. The anchor has a diameter of approximately 150 mm. Unlike larger steel wires used in the prior art, the loose filaments are not stiff enough to remain organized when they are placed in the cavity within anchor 24. It is very difficult to maintain any type of organization while the liquid potting compound is added to the cavity (or when any other type of termination technology is used, with the “spike and cone” frictional type of anchor being another example). The filaments tend to lose the aligned orientation needed to produce a consistent termination. In this potted termination example, the filaments when oriented upward tend to become a disorganized tangle, and are generally inconsistent in alignment. The alignment issue worsens with increasing scale as the filament volume and termination length both increase.
The result is a termination which commonly fails well below the ultimate tensile strength of the cable—obviously an undesirable result. In addition, the disorganized nature of the strands within the cavity produces a substantial variation in strength from one termination to the next. In other words, the process of terminating a large synthetic cable is not predictable nor is it repeatable.
One prior art approach to this problem has been to subdivide the cavity within anchor 24 using some type of insert. The insert subdivides the tapered cavity into several wedge-shaped sections. The available filaments are then divided evenly among the wedge-shaped sections. This approach helps improve certain performance characteristics but does not address the majority of significant processing challenges inherent with large synthetic cables.
The present invention solves the problem of larger cables by (1) dividing the cable into smaller components which are in the size range suitable for the prior art termination technology; (2) providing a collector which reassembles the individual terminations back into a single unit; and (3) maintaining reasonable alignment between the terminations and the smaller cable components while the terminations are “captured” within the collector.
The goal of maintaining alignment between the terminations and the smaller cable components is significant. Some additional explanation regarding the need for good alignment between the strands and the anchors used to terminate them may aid the reader's understanding. FIGS. 9 and 10 illustrate the result of flexing a strand 12 before or during the termination process.
In FIG. 9, strand 12 has been flexed. Jacket 14 has slipped somewhat with respect to filaments 16 it contains. Filaments have also slipped with respect to each other. In FIG. 10, the same strand has been straightened. The reader will observe that some of the filament slippage remains. This is the result of the fact that synthetic filaments have very low stiffness. When they slip relative to one another, there is no significant restoring force. A bend or kink may exist in an individual filament, but little restoring force is produced. For a prior art wire cable, the bending or kinking of a wire produces a significant restoring force. When a wire rope bends it generally returns to the same state once the bend is removed. This is not the case for cables made of synthetic filaments. The alignment issues occur with or without a jacket around the strand. Further, the alignment differential increases as the size of the cable increases. The reader will thereby perceive the importance of keeping a synthetic cable and/or its component strands straight in the vicinity of the end when creating a termination.
It is also important to maintain alignment between a strand and the anchor used to terminate it. The region where the filaments exit the anchor (often called the “neck” of the anchor) is significant. If the freely flexing portion of the synthetic-filament strand is bent with respect to the anchor when loaded, a large stress riser will form in the neck region. The freely flexing portion bends quite easily and it is not able to withstand significant lateral loads without badly reducing the overall strength of the strand/termination. Maintaining the desired alignment for these large cables is a more complex problem—with processing and performance issues increasing with increasing scale. The present invention presents a solution to these problems.
The present invention seeks to improve both processing and performance issues. The main processing advantage of the invention is the fact that it allows the use of well-developed and repeatable “small cable” termination technologies to be used with larger cables. The main performance advantages of the invention result from the fact that “small cable” terminations produce good repeatability and good overall strength, along with the fact that non-uniform loads are decoupled and/or alignment between the strands and their respective terminations are improved. The invention “collects” multiple small cable terminations into a single collector, thereby allowing the advantages of a small cable termination to exist in a larger cable.